The present invention relates to a novel method of determining the filler content of finished paper.
The manufacture of paper involves the use of other ingredients in addition to fibers (paper pulp), for instance such ingredients as sizing agents, fillers, and possibly also pigments. Filler is used to impart special properties to the paper, particularly with regard to its printing characteristics, brightness and opacity. The filler is often cheaper than the paper pulp and is also used to lower the manufacturing costs. The most common fillers in this regard are white mineral substances, such as kaolin, chalk, talc and gypsum. Chalk is normally used in present-day manufacture. The filler is normally added in large quantities, particularly when manufacturing so-called fine paper. The paper may have a filler content of up to 50% ash content, although filler contents of about 20% are common.
Paper is manufactured on a paper machine from stock or stuff consisting of an aqueous solution of mainly fibers and additives, for instance filler. The stock, or stuff, normally contains about 0.5 percent by weight fibers. The stock is de-watered on a wire, to form a coherent paper web. The wire can be compared with a filter cloth which is essentially impermeable to fibers but permeable to the liquid and, e.g., to filler particles that fail to adhere to the fibers. Downstream of the so-called wire section the de-watered paper sheet will contain about 20 percent by weight fibers. The solution that passes through the wire is returned to incoming stock through the medium of a recycling circuit, normally referred to as the short circulation.
The filler particles are normally very small in size and chemicals which function to flocculate the filler and bind it to the fibers are added to the stock, to prevent the filler particles passing through the wire in the de-watering process. These chemicals are normally referred to as retention agents and include, e.g., polymer compounds that carry electric charges. Normally, about half of the filler is retained in the paper sheet and the remainder recycled. Only a relatively small amount of retention agent is added to the stock and it can be assumed that the retention agent remains generally in the paper, or looses its activity.
The term retention has been introduced as a measurement of how much of the material delivered to the paper machine fully remains in the finished paper. When the concentration of incoming material or one of its components, e.g. fibers or filler, in the incoming stock is C.sub.in and the flow is F.sub.in, the weight of the dried paper per unit of time is P and the concentration in the recycled water is C.sub.out, the retention R can be calculated as follows: ##EQU1## Since the recycled material constitutes the difference between the incoming quantity and the quantity of finished paper, the equation can be approximated to the following equation, since F.sub.out .apprxeq.F.sub.in : ##EQU2## Retention is used as a measurement of effectiveness in the de-watering process, and may also be applied as a parameter in controlling surface weight and filler content of the paper sheet.
Retention in the de-watering process can be assessed by on-line measuring of the consistencies of the incoming and outgoing flows. However, measuring of the consistency of the incoming and outgoing flows is encumbered with serious drawbacks, for instance such drawbacks as a disturbing influence of air bubbles, high stock temperatures and uneven consistency distribution in the filtrate, the white water. Sensitivity is also dependent on the measuring method used. One example of known methods applied to this end is the method taught by WO 86/07458, in which two optical measuring devices are used, of which one uses infrared light and the other uses polarized light. These signals are combined and then used to calculate the fiber/solids and filler contents of respective flows.
Another method involves measuring surface weight and ash content (filler content) of the dried sheet further on in the paper machine. This method employs the use, e.g., of traversing measuring instruments which move across the paper web. These instruments apply, e.g., X-ray fluorescence or X-ray absorption to determine the filler content. It is obvious that these known methods cause a delay in the control of stock consistency. Furthermore, when starting-up the paper machine, the paper web does not extend through the entire machine up to the point at which the measurements are taken.
Hitherto, retention has functioned solely as a diagnostic instrument in paper manufacturing processes. Furthermore, retention is not controlled essentially directly when practicing prior art methods. Normally, fibers and filler are metered to the stock in controlled quantities while awaiting equilibrium. Obviously, such indirect control involving addition of material to the large liquid volume constituted by the stock combined with the short circulation will also cause control delays.
There is therefore a need for a simple method by means of which retention can be predicted with a high degree of accuracy and with the smallest possible delay in the de-watering process, and also an associated method for controlling retention quickly and responsively and thereby a novel method for controlling the filler content of finished paper.